1. Technical Field
The present invention generally relates to a method for writing a grating in a large diameter optical waveguide; and more particularly to a method for writing a grating in a large diameter optical waveguide in which photosensitivity is restored or increased in a hydrogen or Deuterium loaded large diameter optical waveguide.
2. Description of Related Art
It is known in the art that the presence of hydrogen, Deuterium or other suitable photosensitizing gases in a germanium doped waveguide enhances the photosensitivity of that waveguide. This is well documented in numerous references such as, “Photosensitive changes in Ge-doped fibers observed by Raman spectroscopy,” D. McStay, SPIE vol. 1315 Fiber Optics '90, pp. 223-233; “Permanent photoinduced birefringence in a Ge-doped fiber,” Francois Ouelette, Daniel Gagon, Michel Poirer, Applied Physics Letters, vol. 58, pp. 1813-1815, 1991; U.S. Pat. No. 5,235,659 “Method of making an article comprising an optical waveguide,” Robert M. Atkins, Paul J. Lemaire, Victor Mizrahi, Kenneth L. Walker; Aug. 10, 1993; U.S. Pat. No. 5,287,427 “Method of making an article comprising an optical component, and article comprising the component,” Robert M. Atkins, Paul J. Lemaire, Victor Mizrahi, Kenneth L. Walker; Feb. 15, 1994.
These prior art descriptions focus primarily on methods for increasing the photosensitivity of fiber. Fiber has several unique characteristics. Optical fiber is typically coated with an organic polymer that cannot withstand high temperatures. Single mode optical fiber is also typically 80 or 125 microns in diameter. These characteristics drive the method by which the above references incorporate hydrogen into the optical fiber. In particular, very high temperatures are not employed, as this would damage the optical fiber coating. However, low temperatures limit the diffusion rate at which hydrogen is incorporated into the glass. For 125 micron optical fiber, this not a terrible problem as at temperatures between 50° C. and 80° C. (well below the damage temperatures for most fiber coatings) significant hydrogen can be diffused into the fiber in a reasonable time frame (less than 1 day). However, for significantly larger glass structures the time frame quickly increases to excessive times.
Photosensitivity requirements can only be analyzed qualitatively at present. It was recognized from early in the development that the requirements were not met in the initial H2 loading iteration. The combination of a low photosensitivity waveguide and a concentration of−1.44×10^20 ions/cm^3 of H2 were not sufficient to allow gratings to be collocated without undesirable out-of-band spectral characteristics. To add a third collocated grating, it was necessary to increase the pressure by a factor of 6 and the temperature was reduced 25° C. The resulting H2 concentration increased to −5.47×10^20 ions/cm^3, as shown in FIG. 1. This higher concentration has now been certified as sufficient. However, the time required to reach 95% saturation, 21 days, is excessive and must be reduced to achieve a reasonable cycle time. The next step will be to determine the effects of raising and must be reduced to achieve a reasonable cycle time. The next step will be to determine the effects of raising the temperature by 15-20% (absolute), to 250-275° C., which would reduce the loading time to 3-4 days and the concentration to 3.46×10^20 ions/cm^3, as shown. Provided that there is still sufficient photosensitivity after the solubility losses, an additional step would be to determine if the pressure could also be reduced without causing the sideband issue to reoccur when collocating gratings.
The plot in FIG. 1 was constructed using the hydrogen diffusion equations for glass and illustrates the time issues more clearly. For cylindrical geometries of 2000 microns, the time to reach a reasonable diffusion equilibrium is many days, perhaps even weeks at low temperatures.